This invention relates to the fractionation of a mixture of fatty acids to obtain a fraction enriched with biologically active polyunsaturated fatty acids.
It is known that fatty acids having different degrees of saturation can be separated from a mixture by formation of an inclusion complex with urea in the presence of a solvent and separation of the liquid fraction enriched with the complex. For example, French patent no. 1 603 383, which corresponds to British patent no. 1 240 513, relates to the enrichment with .gamma.-linolenic acid (triunsaturated) of a mixture of fatty acids emanating from the oil of the evening primrose "Oenothera" which additionally contains fatty acids having a lower degree of unsaturation.
There are also known processes for the selective enrichment with polyunsaturated fatty acids, of which the first double bond is in the .DELTA.6-position (for example .gamma.-linolenic acid), of a mixture of fatty acids additionally containing polyunsaturated fatty acids, of which the first double bond is in the .DELTA.9-position (for example .alpha.-linolenic acid), by complexing with urea under specific conditions which involves separation of the position isomers (see, for example, published European patent no. 178 442). Although this process, which is not continuous, leads to satisfactory enrichment from the point of view of selectivity and yield either on a laboratory scale or on a pilot scale, it is difficult to carry out on an industrial scale. For example, in batches of several t., the formation of the inclusion complex in a tank takes place very slowly. It only works properly, i.e., with acceptable selectivity, with very slow stirring. As a result, the heat exchanges are poor. Now, fine adjustment of the reaction temperature is necessary in order not to cause the redissolution and/or ageing of the urea crystals which would then no longer be capable of including the fatty acids, thereby giving rise to a reduction in the selectivity of enrichment.